Method and bath for coating metal with molten zinc



United States Patent NIETHOD AND BATH FOR COATING NIETAL WITH MOLTEN ZINC John E. Logan, Pittsburgh, Pa., assignor to The Wean Engineering Company, Inc., Warren, Ohio, a corporation of Ohio No Drawing. Application September 25, 1957 Serial No. 686,027

13 Claiins cum-114 zinc wherein the metal to be coated is immersed in a bath containing zinc and sodium.

Various methods of coating base metals with other metals have long been used. Such methods include elec-" troplating of the base metal with the coating metal and v immersing the base metal into the molten coating metal. Specifically, zinc has heretofore been applied to a base metal both by electroplating and by dipping the base metal directly into a bath of molten zinc, otherwise known as galvanizing.

I have discovered new and useful methods and compositions for coating a base metal with pure zinc. I preferably subject the base metal to a mixture comprising zinc and sodium. In a preferred practice of my invention, I heat a mixture of zinc and sodium under a protective atmosphere to a temperature less than the boiling point of sodium. I then immerse the article to be coated in the mixture until his coated with zinc to the desired thickness. Most advantageously, I beat the mixture to form a bath having a temperature of 1000 to 1300 F. If the temperature is higher than this, excessive sodium fumes may be encountered. If the temperature of the bath is increased, the speed with which a given coating will be obtained is also increased. In like manner, agitation of the bath or increased concentrations of zinc tend to increase the speed with which a given coating thickness will be, obtained.

While the speed of coating increases with the temperature of the bath, the advantage obtained in the coating may be offset by the dilficulty of handling the increased sodium fumes. When the temperature of the bath is lowered substantially below 900 F., the coating tends to become spotty and uneven. However, a continuous coating may be secured with bath temperatures at least as low as 750 F. by increasing the time that the article is immersed in the bath and by agitating the bath while the article is so immersed. At temperatures below about 750 F. the immersion time and the amount of agitation required to secure a continuous coating become unduly prolonged.

Preferably, I provide a mixture of to zinc by weight in metallic sodium. A mixture of these proportions will give good coatings. If less than 10% zinc is provided in the mixture, it becomes necessary to keep the article immersed for longer periods to get a continuous coating of the desired thickness. When the zinc concentration falls below about 3% a continuous coating can be secured only by agitating the mixture and by extending the immersion time of the article. If the percentage ice to a temperature of l000 to 1 30Q F., zinc. will drop. out of the mixture and rest upon the bottom of the con-- In other words, the maximum amount of zinc that will stay in solution with the sodium at the pro-1 tainer.

ferred temperature is'about 15 This is apparently the limit of solubilityof zinc in a molten sodium. Any additional zinc which is added to the bath will drop out as pure zinc. Whenthe article is immersed in a bath in. which the zinc concentration is above the apparent limit, of solubility, that part of the article which is immersed only in the upper part of the bath will be coated with zinc as heretofore described. The part of the article; which is immersed in the lower part of the bath, how

ever, will be coated with spangled zinc which is char acteristic of galvanizing. Whereas the coating from the upper part of the bath will be of apparently pure zinc and will be ductile, the coating from the lower part of.

the bath will be a zinc-iron alloy and willbe brittle. The invention may, of course, be practiced by immersing the metal only in the upper part of the bath. The presence of zinc which has dropped out in the lower part will insure that the zinc in the upper part of the bath is at the limit of solubility, thus promoting the speed and efliciency of the coating process.

In the ordinary galvanizing process the coating of Zinc which is applied by hot dipping is spangled and brittle.

The coating is brittle, and if the article is bent or deformed the zinc coating will crack. My invention produces a ductile zinc coating similar to that which is obtained by the electroplatingof zinc. The coating is a dull layer of apparently pure metallic zinc applied to the 1 base metal. While I am able to produce a product that is comparable to that produced by electroplating zinc, it 3 is obvious I am able to avoid the use'of complex electro- 5 plating equipment. I

It is also apparent that immersion in the bath will add heat to the article which is being coated. This may have the effect of preparing the article forsubsequent annealing or hardening by slow or rapid cooling from an elevated temperature. It will be apparent that various efiects may be obtained by use of a'bath at varying temperatures.

My invention may perhaps be best illustrated. by refer-i ence to several examples.

In one series of tests, a mixture of 10% zinc and sodium by weight was heated to a temperature of 1040 ".11 F. Steel strip was cleaned by acid pickling and then, rinsed and dried. Following. this it was inserted inthe bath through a protective atmosphere. In some instances the bath was stirred or agitated. Upon withdrawal, each side of the strip was found to be coated with a ductile Theincrease in thickness was measured by a micrometer,-one-half of the total increase;

matte layer of zinc.

being the coating thickness on each side.

Other tests indicate that the proportions of the bath,

Pa M f- 2 19.69.

an em a 3 tsmtzsrah rss. and z c-s dium .ratiq beratin t9v secure desirable coating conditions. The following series of tests was run under the same general conditions as heretofore. 'Ihe bath was composed of zinc and sodium and was stirred while the strips were immersed.

Immersion Example Tempera- Condition of Coating tur fF- Continuous.

Spotty, discontinuous. O t m tl Example Immersion Tempera- Bath Gondition of jme tune, F. Coating 157211-.-- 1 e seconds 1,000 {58 7; Na Piggy but (stirring). 27% K Ad it amo t o not s t mres l d n a ene al less satisfactory coating.

While I have described a present preferred embodiment of my invention, it is to be distinctly understood that I do not so limit myself and that my invention may be otherwise practiced within the scope of the following claims.

I claim:

1. lfhe method of coating a metal with zinc which comprises heating a mixture of zinc and sodium containing notil'ess than 10% nor more than 15% zinc with the balance being substantially pure sodium to a temperature between 1000 F. and 1300 F., protecting the bath from the atmosphere, and immersing the metal therein until it is coated with zinc.

2. A bath for coating a ferrous metal with a ductile layer of zinc which comprises a mixture of zinc and sodium containing not less than zinc nor more than in t alance being s bstan a P s ium heated to a temperature of 1000 F. to 1300 F.

3.- Th? m hpdr f oati a er u sta w h p? which comprises heating a mixture of zinc as solute in dium a ad entv whi in net 9 th n 3% inc t form a bath, protecting the bath from the atmosphere, d imsr i t me a di sctl the at unt i i oat d it Zin t e od um b in Pre en i eu i cn i men t Pr u e a u t e l e oft-in ns smooth unspangled surface.

4. The method of coating a ferrous metal with zinc which comprises heating a mixture of 3%v to 15% zinc and 85% to 97% metallic sodium, exclnsive of impurities, to form a molten bath protecting the bath from the atmosphere, and immersing the metal therein until it is coated with a ductile unspang led layer of .5. The .msth s 9f coatin lie rpus metal h n which comprises heating a mixture of 10% to 15% zinc and 85% to 90% metallic sodium, exclusive of impurities, to a temperature between 1000 F. and 1300 F.,

5 protecting the bath from the atmosphere, and immersing the metal therein until it is coated with a ductile unspangled layer of zinc.

6. A bath for coating a ferrous metal with a ductile unspangled layer of zinc "which comprises a mixture of 10 zinc and sodium containing from 3% to 15% zinc and the balance being substantiallypure sodium heated to form a molten bath.

7. A bath for coating a ferous metal with a ductile layer of-zinc, which bath comprises a mixture of zinc and 15 sodium, containing not less than 3% Line nor more than 15% zinc, the balance being substantially pure sodium, heated-to a temperatureof 750- Pvt 0,130 0 8. A bath for coating. a ferrous metal with a ductile layer of zinc, which bath notrlprisfes a mixture of zinc and sodium, comprising not less than 3% -zinc nor more than 15% zinc, the balance being substantially pure sodium, the bath beingv heated: to atemperature not lessthan 90.0 F.

9. The method of coating a metal. with zinc which comprises immersing the metal in ,a bath comprising zinc as solute in molten sodium as solvent, the bath being heated to a temperature of. not less than 759- F.

10. The method of coating. 21. metalv with zinc which comprises immersing themetal in a bath comprising. zinc as, solute in molten sodium as. solvent, the bath 'being heated to a temperature. between, 100.0 E. and 1300? F. 11. The methodof coating a ferrous metal. with zinc which comprises immersing the metal. in a bath comprising 'zinc as. solute in molten sodium as solvent, the bath being heated to a temperature of; not less than 750-" F., and comprising at least 3% zinc, the balance being substantially sodium, exclusive of. impurities.

1 2. The, method: of: coating. a metal with. zinc which comprises heating a bath comprising zinc as solute in 40 sodium as solvent, introducing the. uncoated metal therein until it is coated with zinc, and then withdrawing the metal therefrom.

13-. The method of coating a. ferrous metal with zinc which comprises introducing. the uncoated metal into a bath of zinc as solute in molten sodium, as solvent, maintaining the ferrous metal therein until it is coated with zinc, and then withdrawing the ferrous metal therefrom.

Reisrenus G d the me oftni patent UNITED STATES A EN S- 293,886 R p. 19, 1884 514.958 Feb. 20', 1 9 622.099 M 2. 1899 as 1,994,275 Mar.1 2,1935 2,27s,2 32 Mar. 10, 19 42 

1. THE METHOD OF COATING A METAL WITH ZINC WHICH COMPRISES HEATING A MIXTURE OF ZINC AND SODIUM CONTAINING NOT LESS THAN 10% NOR MORE THAN 15% ZINC WITH THE BALANCE BEING SUBSTANTIALLY PURE SODIUM TO A TEMPERATURE BETWEEN 1000*F. AND 1300*F., PROTECTING THE BATH FROM THE ATMOSPHERE, AND IMMERISING THE METAL THEREIN UNTIL IT IS COATED WITH ZINC. 